The flip side of flying

Early aircraft designers would boast, “Give me enough power and I can make a barn door fly.” And it’s true. (Illustration by John Lewis)

Wednesday, March 20, 2013 - 10:15am

The recent movie Flight stars Denzel Washington as a flawed-hero airline pilot. The film has drawn some criticism among aviation types for depicting an airliner flying upside down. I’ve seen only a short clip of the scene, so I’m not sure how true-to-science it is, but I can say that the apparent contradiction of inverted flight is one that is widely misunderstood—even among pilots.

In primary flight training, all aviators learn the Bernoulli principle of lift, named for 18th century Swiss physicist Daniel Bernoulli. I’ll spare you the gory physics, but the gist is that the curved upper surface of the wing forces the air to move faster and “stretch out” the molecules, creating lower pressure on top. The higher pressure of the “unstretched” air underneath pushes the wing upward, and presto, the magic of flight.

Others point to the (Isaac) Newtonian theory of “for every action there is an equal and opposite reaction.” Tilt a wing of any shape upward against the air underneath and the reaction will be to push the wing skyward. The angle of tilt is called the wing’s “angle of attack.” Whatever is attached to the wing will go along for the ride. Baby robins—using only their tiny bird brains—discover this every spring as they leave the nest. Anyone with a human-size brain who has held a hand outside the window of a moving car and tilted it upward has also felt this principle in action—or “reaction,” as the case may be.

So which theory applies—Bernoulli’s or Newton’s?

Spoiler alert: the answer is both.

Bernoulli’s principle is alive and well, unlike Bernoulli himself (or Newton, for that matter). Yes, there is a distinct lessening of air pressure atop a curved wing. The Wright Flyer, Piper Cubs and other early flying flivvers had sweeping, teardrop-curved airfoils that produced enormous amounts of aerial buoyancy, but at the cost of great drag. Modern aircraft wings often use drooping leading edges (slats) and trailing edges (flaps) to mechanically alter the wing’s shape on takeoff or when slowing down for landing. You can turn a high-speed jet into a closer relative of the Piper Cub by deploying such retractable wing ­re-shapers when slow flight is desirable.

But the Bernoulli effect really just complements the idea that if you simply push a flat wing against the air at an upward angle, the air will lift the wing. It optimizes the lift created by pushing down on the air beneath the wing. Early aircraft designers would boast, “Give me enough power and I can make a barn door fly.” And it’s true. The tiny, uncurved wing of the 1950s’ high-power F-104 jet fighter proves the point.

For additional proof of the Newtonian theory of lift, you need look no further than the balsa gliders that every kid has flown. Their wings are flat, but they fly.

I had an amusing conversation on the radio with Tom and Ray Magliozzi, better known as “The Tappet Brothers” of the NPR show Car Talk.” They asked me how it is that airplanes can fly upside down, since the Bernoulli principle seems to make that a physical impossibility. I started explaining how angle of attack also creates lift; how flying upside down is far less efficient, but possible; how some airshow pilots’ airplanes even have symmetrical wings so they can do it better; and finally I asked, “Do you really want to hear all this?” I was getting a little bored, myself.

The bottom line is that a wing can work its magic upside down, but unless it’s explicitly designed to do so, it will fly a lot less efficiently. Of course, all of this conversation about the wing doesn’t address other difficulties, such as how well an upside-down fuel system operates and what happens to the oil distribution—not to mention who’ll clean up the contents of the bar.

Plain text

Quote/Unquote

““When I made the film The Invention of Lying, they gave me a private jet for getting back and forth between New York and London. I thought, ‘I will never use it’ but I ended up using it every weekend. You turn up, right, and the airport is completely empty. I mean, there’s just someone at the desk and then the pilot, who says, ‘Are you ready to go?’ and you say, ‘Don’t you want to see my passport?’ and he goes, ‘Oh yeah, I suppose I’d better.’”
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